Currently, various recording materials have been studied and developed for practical use in the field of information recording.
Among these recording materials, thermosensitive recording materials have the following advantages: (1) a simple image recording is realized only by a heating process, and (2) mechanisms of a device is simple, downsizing thereof is easily achieved, and a recording material is easily handled and at low cost.
Accordingly, thermosensitive recording materials are used in a wide range of fields, including an information processing field (output of an electronic desk calculator, or a computer), a field of recorders for medical instrumentation, a field of low speed and high speed facsimiles, a field of automatic ticket benders (for railway tickets, or admission tickets), a thermosensitive copying field, a field of labels for a POS system, and a field of tabs. For example, to satisfy the properties suitable for various uses, such as a label to be adhered to a surface of a moisture vapor resistant wrapping material, and a label capable of peeling after adhered once, thermosensitive recording materials are required to satisfy various properties, such as physical strength resistant to bending or breaking, dimensional stability, and water insolubility, at the same time. From these reasons, synthetic paper or synthetic resin films are often used as a support of a thermosensitive recording material.
In the fields of labels for POS system and tabs, especially, thermosensitive recording materials are often used with an adhesive layer provided on a back side thereof. There are diverse range of the adhesives used for the back side, such as an adhesive intended for permanent bonding, an adhesive intended for releasing and re-adhering, and an adhesive capable of exhibiting adhesion at low temperature, which is used for a label for frozen food products.
In the case where synthetic paper or plastic film having higher smoothness than that of paper is used as a support, curling of a thermosensitive recording material may occur, especially when an adhesive layer is provided thereto. When a back layer is provided between a support and an adhesive layer, moreover, adhesion between the back layer and the support reduces, which may cause peeling from the portion at which the adhesion is weakened.
It has not yet been fully discovered why peeling or curling of a thermosensitive recording material occurs, but it is assumed that low molecular weight components, such as a plasticizer, a tackifier (a tackiness imparting agent), and an emulsifier, contained in an adhesive used in an adhesive layer migrate into a back layer or a support, in the case where a support is synthetic paper or a plastic film, to thereby reduce adhesion to with the back layer, or to thereby impart plasticity to one side of the support.
PTL 1 discloses that a back coat layer is provided on a back surface of a paper support of thermosensitive recording paper, where the back coat layer contains a core-shell acrylic emulsion and an inorganic pigment having oil absorption of lower than 50 mL/100 g, to improve adhesion of an adhesive layer to the back coat layer, and water resistance. This back coat layer however has problems that film forming ability and barrier properties are insufficient, and peeling or curling may occur when the support is synthetic paper or a plastic film.
Moreover, there is disclosed that the back coat layer is cured by a curing agent. However, this description merely teaches that the curing agent can be used for the purpose of enhancing water resistance of a cellulose base support, but not for preventing reduction in adhesion strength due to organic materials, which are easily migrated, contained in an adhesive or support.
PTL 2 discloses that a paper support is coated with a thermosensitive layer and a protective layer, followed by coated with a back coat layer to correct the curl formed during the formation of the thermosensitive layer and the protective layer. Moreover, there is a description that a core-shell acrylic resin can be used in the back coat layer, but the core-shell acrylic resin is used therein for the purpose of improving water resistance of hydrophilic paper, not for preventing a problem caused by migration of organic materials in an adhesive or support.
In the case where the strength of the back coat layer itself, and adhesion between the back coat layer and the support are insufficient, pealing or curling of the thermosensitive recording material using synthetic paper or a plastic film as the support partially occurs as time passes.
Moreover, we have previously proposed in PTL 3 that a back layer is provided between a back surface of a support and adhesive layer of a thermosensitive recording material, where the back layer contains non-foamable plastic hollow filler, a styrene-acryl copolymer having an acid value of 200 or greater and Tg of 80° C. or higher, and an acrylic emulsion resin, and is reinforced with potassium zirconium carbonate, and ammonium zirconium carbonate. This technique is to prevent peeling between the back layer and the adhesive layer by a crosslink reaction with an acrylic adhesive in the adhesive layer, not directed to prevention of peeling between the back layer and the support.
Further, the back layer of this thermosensitive recording material has barrier properties to some extent, but PTL 3 does not tech or suggest a problem caused by volatile organic materials contained in the adhesive. Therefore, such thermosensitive recording material cannot be used for various purposes, and curling thereof may occur due to partial peeling, depending on an adhesive for use.